U.S. Pat. No. 4,647,817, Fahnrich et al, the disclosure of which is hereby incorporated by reference, assigned to the assignee of the present application, describes a circuit for low-pressure fluorescent lamps in which the lamp is operated at high frequency, for example in the order of about 45 kHz. The lamp has an ignition circuit connected in parallel thereto and serially with the heater circuit, that is, with first and second electrodes, which includes a capacitor and a parallel connected positive temperature coefficient (PTC) resistor. After preheating of the electrodes by current flowing through the initially cold PTC resistor, a voltage rise will occur across a resonant capacitor and an inductance, causing ignition of the lamp.
When voltage is applied to the circuit, the PTC conductor is of low resistance and thus permits preheater current to flow through the electrodes of the lamp. This current heats the PTC resistor and, in accordance with a time-heater curve characteristic to the particular PTC resistor, it will assume a high resistance state. This, effectively, interrupts preheating of the electrodes and the lamp will fire or light due to the resonance circuit. The PTC resistor ensures adequate and rapid preheating of the electrodes and reliable ignition of the lamp.
In the circuit, as proposed in the referenced U.S. Pat. No. 4,647,817, it is a disadvantage that the PTC resistor, after-ignition and during the entire operation of the lamp, will carry current, and will be heated. Due to the long lifetime of fluorescent lamps, considerable energy will be wasted for heating the PTC resistor. Continued heating of the PTC resistor also applies an additional thermal load on the overall circuit, which may lead to failure of a circuit component, and especially to failure of the PTC resistor itself. Additionally, when fluorescent lamps are installed in locations requiring air-conditioning, the cooling effect of air-conditioning must additionally remove heat generated by the PTC resistor of the fluorescent lamp.